I think there are two plausible definitions of disease (aka pathology):
1. A condition which reduces probable reproductive success - this is an objective, albeit probabilistic, biological definition.
2. A condition which causes pain, suffering, distress - this is a subjective, personal and to some extent social definition.
So that lung cancer, or coronary heart disease, or pneumococcal pneumonia are biologically diseases because they threaten life, reduce the prospects of reproduction and the ability to assist in guarding life and assisting the reproduction of the family and group.
While most forms of pain - such as headache and backache, fever, vomiting and diarrhoea, also psychological symptoms such as depression and anxiety, and probably even deformity and disfigurement, are 'diseases' mainly because they cause suffering. In fact they are usually called 'symptoms' rather than diseases - and they are the kind of things that lead the sufferer to seek treatment.
(A 'symptom' is something the patient 'complains of' in the terminology - as contrasted with a 'sign' which is observed by the doctor. A sign would be a heart irregularity or lung crackles heard with a stethoscope, or a deep abdominal swelling palpated with the hands.)
In fact, many of the subjective symptoms reduce to the biological definition because the personal suffering leads to impairment of functioning. For example a severe headache leads to stopping work and becoming - for a while - dependent and less able or willing to preserve life and pursue reproductive success; and also less able to contribute to social function.
But, by the biological definition, disease has a larger scope than usually noticed. It includes everything which impairs potential and probable reproductive success.
By this interpretation 'disease' includes many almost universal features of modern life. For example, any personal motivation or sexual preference which impairs reproductive success.
Because it is characteristic and indeed universal of modernized societies (The West) that by personal decision, almost all people - on average, en masse - have chosen, that is are motivated to choose, significantly sub-replacement reproduction.
This chosen sub-fertility is therefore - in its many causes and justifications - objectively a disease state.
It is indeed an epidemic disease state.
And this is not a matter of subjective opinion - but a plain and obvious biological fact,